Camelina sativa (L.) Crantz is grown for its seed, the oil of
which may be used for cooking, biofuel, or as a lubricant, and the crop has
potential as an alternative to canola. Camelina is relatively new to the Pacific
Northwest and is currently being grown on a small scale in Oregon, Washington,
Idaho, and Montana.

Fig. 1. Camelina plant affected with downy mildew. Note the white sporulation on the undersides of the leaves. Affected leaves become necrotic with age.

During a field trial in Oregon, twisted stems, stunting, and poor
flowering were observed in May 2009 on plants seeded in late September 2008.
Symptomatic plants showed an abundant white woolly growth on the undersides of
leaves and on stems (Fig. 1). Symptoms were also observed on the upper stems,
flowers, and seed pods following rain events in early June (Fig. 2). Sporulation
of a downy mildew organism was present. Conidiophores (Fig. 3) were hyaline,
straight, measured 11.2-19.2 µm at the widest, and tapered slightly from the
region of the first branch down to the base, which was occasionally faintly
bulbous. In length conidiophores ranged from 192-332.8 µm, were monopodially
branched, and had (3-)4-6(-7) branches terminating in recurved tips. Terminal
branchlets averaged 3.2 µm wide at the base, and ranged from 14.4-24 µm in
length. Conidia were broadly ellipsoidal, and measured 19.2-24.3 × 25.6-26.9 µm.
Oogonia, occasionally found in stems but abundant in leaf tissues, were
irregularly round to oval, somewhat angular, and dark amber with age (Fig. 4).
The oogonia measured 44.8-65.3 × 44.8-51.2 µm. Oospores were pleurotic, with a
mean diameter of 33.7 µm (28.8-38.4 µm).

Fig. 2. Upper stem of camelina white with sporulation.

Fig. 3. Conidiophore of H. camilinae. Bar = 10 µm.

Fig. 4. Oospores of H. camilinae in stem tissue. Bar = 10 µm.

Sporulating leaf tissue was ground and total DNA was extracted
using a FastSpin kit (MP Biomedicals). The mRNA ITS region was amplified using
primers DC6 and ITS4 (1) and sequenced. The nucleotide profile obtained was
compared to those in GenBank and gave 100% identity to Hyaloperonospora
camelinae strain MG (accession AY531456.1) over 566 nucleotides.

Three-week-old greenhouse-grown camelina plants were inoculated by
rubbing sporulating stem sections on moist leaves and enclosing the stem piece
and plant in a plastic bag to maintain humidity. Six plants were inoculated and
maintained on the greenhouse bench under shade cloth. Two plants were not
inoculated but were wetted, bagged, and maintained similarly. Sporulation on
inoculated plants occurred after eight days. All six inoculated plants became
symptomatic; neither of the control plants became diseased. DNA from affected
leaves was extracted and sequenced as described above, and again gave 100%
similarity to H. camelinae strain MG.

Downy mildew on camelina has been reported in Minnesota (3), where it was identified as Peronospora camelinae (now Hyaloperonospora camelinae), although no supporting evidence was given. In Montana, a downy mildew on camelina was identified as P. parasitica (now Hyaloperonospora parasitica)(2). To our knowledge, this is the first time H. camelinae has been confirmed in the western USA.
Severely affected plants
produce little or no seed, and susceptibility to downy mildew may limit the
areas in which camelina can successfully be grown.